Gas separating and venting filter

ABSTRACT

A fluid filter is disclosed which separates gas from liquid and vents the separated gas from the filter. The filter includes a vented housing through which the fluid stream passes. Liquid-wetting filter means carried in the housing in the path of the fluid stream permits the passage of liquid only. Gas separated from the fluid is vented through vent opening means which is covered by a liquid-repellent filter to permit the passage of gas only. An automatic pressure sensitive control means may be used to seal the vent opening means against the entry of ambient air but to automatically release separated gas from the filter. The liquid-repellent filter may be secured to the housing by a mechanical bond between the housing and a fibrous backing carried by the filter, or a continuous band of medical grade tape may be used to attach the filter to the housing.

This is a division, of application Ser. No. 002,689 filed Jan. 11, 1979now U.S. Pat. No. 4,238,207, which is a division of application Ser. No.856,147 filed Nov. 30, 1977 now U.S. Pat. No. 4,190,426.

The present invention relates generally to fluid filters which separategas from liquid in a fluid stream and vent the gas. More particularly,the invention relates to such filters which are capable of separatinggas from liquid in fluids which are administered to a living subject.

Fluid filters which utilize a porous membrane or other filter mediawhich becomes resistant to the transmission of gas when wetted by liquidhave been used in a wide variety of situations. One increasinglyimportant use for such filters is in filtering blood, plasma, parenteralsolution or other fluid, as the fluid is administered to a livingsubject, usually human. In the administration of such fluids, inaddition to filtering out particulate matter and potentially harmfulmicro-organisms, it is preferred that any gas or air suspended orentrained in the fluid also be removed so as to eliminate any hazard ofembolism from air or gas reaching the patient.

Present filters which have been used to remove gas from intravenousfluid have often included a liquid-wetting filter membrane which, whenwetted, is resistant to the transmission of gas having a pressure belowthe so-called "bubble point", in combination with a liquid-repellent ornon-wetting filter membrane through which the separated gas must passbefore it is vented through openings in the filter housing. Forfiltering an aqueous solution, such filters are usually referred torespectively as hydrophilic (water-wetting) and a hydrophobic(water-repellent). A hydrophilic filter permits water to pass but isresistant to the passage of gas when wetted by water. A hydrophobicfilter permits gas to pass but is resistant to the passage of water. Theliquid-repellent filter membrane through which escaping gas must passnot only prevents leakage of liquid through the vent openings, but also,by its microporous construction, protects the filtering liquid againstcontamination from the ambient atmosphere. One example of a filterconstructed with wetting and non-wetting filter membranes is shown inU.S. Pat. No. 3,523,408 to Rosenberg. There, the fluid is introducedbetween a pair of equally large, facing filter membranes, one of whichis liquid-wetting and the other of which is liquid-repellent. Gas whichis removed from the fluid by the liquid-wetting filter passes throughthe liquid-repellent filter and vents through open ports in the housing.Other filters of somewhat different construction but similar operationmay be found in U.S. Pat. Nos. 3,854,907, 3,803,810, and 3,631,654.

Filters which have been made as described above, however, have severalshortcomings. If the liquid-wetting filter is broken or otherwise failsso that it permits gas to pass with the liquid, ambient air can beaspirated directly through the vent openings and the liquid-repellentfilter and into the liquid which is being administered, possiblyresulting in embolism in the patient. This may occur, for example, whenthe filter is connected in such a manner that a column of liquid candevelop below the filter and create a suction in the filter. Also, theuse of equipment such as pumps, in an administration therapy set maycreate a suction within the filter housing in certain situations.

Another hazard with the aforedescribed filters is that failure of theliquid-repellent filter would expose the solution or fluid tocontamination from the ambient atmosphere.

Because of these hazards, some filters are constructed whichintentionally permit passage of gas or air entrained in the solution,thereby avoiding the need for vent openings or a filter membrane exposedto the exterior. One such filter is described in U.S. Pat. No.4,004,587. In those filters which have used a vent opening with a filtermembrane in the path of venting gas, some have employed a seal over thevent opening to preclude the admission of outside air into the filter.However, the seals now in use must be manually removed or manipulatedfrom time to time to permit separated gas to vent from the filter.Otherwise, separated gas will accumulate and may eventually block theflow of liquid through the filter. Manual venting is not preferredbecause it requires continual monitoring by an attendant or the patient,which is not always possible or practical. In addition, many of theseseals are subject to being disabled or overridden by the attendant orpatient so that the vent is always exposed to the exterior, with thepotential hazards discussed above.

One filtration unit which uses a manual valve for priming or forreleasing entrapped air is described in U.S. Pat. No. 3,954,623. There apush-button type resilient valve extends through a single vent openingin the housing and has an enlarged bulb inside the housing which sealsagainst the edge of the vent opening. By pushing the valve, the bulb isforced away from the vent, permitting entrapped air to escape. Thefilter in U.S. Pat. No. 3,954,623 however, does not have aliquid-repellent filter to prevent liquid from escaping through thevent, rather it has a synthetic fiber batt between the valve and thefilter housing. If wetted by escaping fluid during priming or routineventing this batt may collapse and, no longer exerting force between thehousing and the valve, permit leakage of fluid from the filter housing.

Present filters have also utilized relatively complex or inconvenientmethods for mounting filter membranes in the filter housing. This isparticularly true of the non-wetting filter membrane, which is oftenconstructed of polytetrafluoroetheylene, perhaps better known under thetrademark TEFLON. Because of its high melting point,polytetrafluoroetheylene is not compatible, e.g., for heat sealing, withfilter housing materials of lower melting points. Thus, to adequatelyseal present filters, it has been necessary in many filters to useadhesive-type bonding which requires very careful assembly operations,or silicone ring seals tightly clamped around the periphery of thefilter membrane. A filter with a silicone seal is illustrated in the'408 patent to Rosenberg. Such construction, of course, increases costsfor materials as well as for labor in making the filter.

Accordingly it is a general object of the present invention to provide agas separating and venting filter which does not suffer from theshortcomings described above. Another object is to provide a gasseparating filter which permits separated gas to vent from the filterthrough a non-wetting membrane but does not permit ambient air to enterthe filter and does not require continual monitoring. A further objectof the invention is to provide more simple means for mountingnon-wetting filters in the filter housing.

These and other objects are met by the present invention by providing afilter which employs a liquid-wetting filter to separate entrained gasfrom liquid and a liquid-repellent filter over vent openings to permitthe separated gas to vent while preventing passage of liquid through thevent openings. Automatic vent control means, sensitive to thedifferential pressure between the trapped gas and atmospheric pressure,may be employed adjacent to the vent openings to automatically venttrapped gas from the filter and to seal the vent opening against theadmission of outside air into the fluid stream at all other times.

For an automatic vent control means, this invention preferably employs aflexible, resilient umbrella valve with a canopy which overlies aplurality of vent openings. The flexibility of the canopy permits it toflex away from the vent openings upon application of pressure from theseparated gas inside the filter housing which is greater than theambient atmospheric pressure. After the gas is vented, the resilientcanopy reseals the vent openings. When less than atmospheric pressureexists in the filter housing, that is, when there is a negative pressuredifferential, the canopy is drawn even tighter against the vent openingsto even better seal them. Manual venting is also provided for by arecess adjacent to the vent openings and below the canopy. By depressingthe canopy into the recess, the canopy portion over the vent openings iscaused to invert or flare away from the vents, permitting the release ofany separated gas which did not have sufficient pressure toautomatically raise the umbrella canopy.

Also in accordance with the present invention, a relatively smallliquid-repellent filter membrane of higher melting point than the filterhousing is provided with a fibrous backing layer and is mounted againstthe inside of the housing, over the vent openings, by bonding thefibrous layer to the housing. The housing beneath the periphery of theliquid-repellent filter is heated until it melts sufficiently to fillinterstices in the fibrous backing. After cooling, the plastic in theinterstices hold the filter and fibrous backing securely to the housing.The liquid-repellent filter may also be simply attached to the housingby a continuous band of medical grade tape which overlaps the peripheraledge of the filter membrane and the adjoining portion of the housing. Byheating the tape and applying pressure theragainst, the tape is causedto more securely adhere to the filter and housing and at the same timestresses in the tape are relieved.

The present invention is more specifically set forth in the followingdetailed description and the attached drawings, of which

FIG. 1 is a vertical cross-sectional view of a filter embodying thepresent invention;

FIG. 2 is a sectional view taken along lines 2--2 of FIG. 1;

FIG. 3 is a top plan view of the filter of FIG. 1, but without theumbrella valve of FIG. 4;

FIG. 4 is an enlarged cross-sectional view of the umbrella valve used inthe filter of FIG. 1;

FIG. 5 is a partial sectional view of a filter embodying the presentinvention, showing the preferred method for attaching theliquid-repellent filter;

FIG. 6 is a sectional view of the filter of FIG. 1, illustratingautomatic venting of gas from the filter;

FIG. 7 is a sectional view of the filter of FIG. 1, illustrating manualventing of gas from the filter;

FIG. 8 is a bottom plan view of the filter of FIG. 1;

FIG. 9 is a partial sectional view showing an alternative method forattaching the liquid-repellent filter.

Turning now to a detailed description of the present invention, as shownin its preferred embodiment in the attached drawings, the presentinvention is embodied in a generally disc-shaped filter housing 10,which has a shallow base portion 12, and a cap 14 covering the baseportion to form an internal chamber. The housing may be constructed fromany material which is compatible with the fluid being filtered, but thematerial is preferably clear, so that the filtering action may beobserved. For filtering parenteral solutions, the housing is preferablymade of clear, rigid, methyl-acrylic type plastic, such as thatavailable under the trademark PLEXIGLAS, Type DR, from the Rohm and HaasCo. of Philadelphia, Pa., U.S.A. This material is relatively inexpensiveand easy to mold.

The shallow base portion 12 of the housing is of one-piece, integral,molded construction, with a flat, circular bottom wall 16 and a shortside wall 18 extending perpendicularly from the periphery of the bottomwall.

For permitting flow into and from the housing, opening means in the formof inlet and outlet ports 20 and 22, respectively, are provided in theside wall 18 of the base portion 12. The ports are located 180° apart,on opposite sides of the base portion, so that the filter may be used ina vertical or hanging position, as would be found in a gravity-flowparenteral fluid administration set. Each port has a slightly taperedbarrel, which extends outwardly from the side wall, and tapers towardthe end for slidably receiving connector tubing or the like. A reverselytapered bore for fluid flow extends through the center of each barrel,and through the side wall.

For mounting a liquid-wetting filter membrane 24 in the base portion, aflat shoulder 26, generally parallel to the bottom wall 16, is providedcontinuously along the inside of side wall 18. The liquid-wettingmembrane is flat, disc-shaped, and of the appropriate size so that theperipheral edge of the filter overlaps the shoulder. The edge of themembrane is then sealed to the shoulder by adhesive, heat-seal,high-frequency welding, or other available attachment means, such asusing the edge of the cap 14 to clamp the filter in place.

In the preferred embodiment, for use in filtering aqueous parenteralsolutions, the liquid-wetting filter membrane is hydrophilic and made ofmixed esters of cellulose, which is compatible with the housing materialand the aqueous parenteral solutions. The membrane has a mean averagepore size ranging from about 0.1 to about 0.45 microns, and preferablyabout 0.22 microns to remove bacteria from the liquid being filtered.Such a filter material is wetted by water and resistant to thetransmission of gas therethrough, so long as the gas pressure is belowthe bubble point of the material. The bubble point generally describesthe differential pressure across the membrane at which gas will beforced through the wetted filter membrane, and, for the preferredmaterial described above, is about 55-60 psid. This material iscommercially available from the Millipore Corporation of Bedford, Mass.,as Type GS Hydrophilic.

This preferred hydrophilic filter membrane 24 is attached to the baseportion 12 by direct heat sealing before the cap 14 is attached. Theflat, disc-shaped filter membrane is positioned in the base portion withthe edges overlapping the shoulder 26. An annular heat-sealing surfaceor head, with a temperature substantially higher than the melting pointsof the filter membrane and the housing is then pressed against themembrane along the shoulder until the membrane and shoulder have meltedsufficiently to coalesce and form a uniform seal around the periphery ofthe filter membrane.

So that liquid flowing through the housing will pass through theliquid-wetting membrane 24, the inlet and outlet ports 20 and 22 arelocated on opposite sides of the shoulder 26, and, therefore, onopposite sides of the filter membrane 24. Specifically, the inlet port20 is positioned in the portion of the side wall 18 above the shoulder,in other words, between the shoulder and the cap 14. The outlet port 22is located in that portion of the side wall below the shoulder, betweenthe shoulder and the bottom wall 16.

As best seen in FIG. 2, the base portion 12 of the housing 10 furtherincludes a series of spaced, parallel ribs 28, which underlie and helpsupport the relatively fragile liquid-wetting filter membrane 24. Theribs project perpendicularly from the bottom wall 16, and are ofgenerally rectangular cross-section, each having a pair of parallel sidesurfaces 30 and a flat-top surface 32. The top surface is located justbelow the shoulder 26, so that it closely underlies the filter membraneand supports it against the upstream pressure of liquid being filtered.Otherwise, without support, the fragile filter membrane may over-flexand tear or burst from the pressure of the filtering liquid.

Channels formed between adjacent ribs 28 conduct filter liquid to theoutlet port 22. As noted earlier, the base portion is generallycircular, and as best seen in FIG. 8, the ribs run parallel to animaginary diametrical line between the inlet and outlet ports. The ribsdo not extend completely across the base but terminate at locationsspaced from the portion of the side wall on the outlet side of thehousing. With this construction, liquid which filters through themembrane 24 is conducted to the outlet port via channels which areclosed at the inlet side of the housing by the side wall 18, and open atthe other end to a semicircular fluid passageway (indicated by arrows inFIG. 8) on the outlet side of the base portion which directs the liquidtoward the outlet port.

The base portion 12 of the housing 10 is closed by the cap 14 which hasa generally flat circular top wall 34 and a continuous peripheral flange36 extending perpendicularly from the top wall for insertion into thebase portion and of appropriate size to snugly engage the inside of theside wall 18. The cap is preferably sealed to the base portion by sonicwelding, which is well known in the art, but other means may be used,such as adhesives or direct heat-sealing.

After the cap is attached, it can be seen that the housing is dividedinto two subchambers by the liquid-wetting filter membrane--a firstsubchamber 37 between the membrane 24 and the cap 14 and secondsubchamber 39 between the membrane 24 and the bottom wall 16 of the baseportion 12. The inlet port 20 communicates with the first subchamber fordirecting fluid which may have both liquid and gas components thereinto.The liquid-wetting filter blocks the flow of gas, and thus only liquidis conducted into the second subchamber from which it exits through theoutlet port 22.

For venting gas or air which is separated from fluid passing through thefilter from the first subchamber, vent opening means in the form of fourvent ports 38 are provided in the top wall of the cap, which is upstreamof the liquid-wetting filter. The vent ports are located in a closelyspaced relationship between the center of the cap and the inlet port 20,and preferably as close to the inlet port as possible, so that when thefilter is used in a hanging position as shown in FIG. 1, separated gasor air which naturally rises from the liquid-wetting filter will be ableto vent from the housing. This location of the vent ports makes thefilter position sensitive in that the vents are preferably locatedhigher than most of the housing.

To prevent liquid from escaping through the vent ports 38,liquid-repellent filter means such as a liquid-repellent filter membrane40 is secured to the inside of the cap 14 over the vent ports. A varietyof liquid-repellent filter membrane materials may be used, but forfiltering aqueous parenteral solutions the membrane is hydrophobic andpreferably made of polytetrafluoroethylene which is naturally waterrepellent. The pore size must be sufficiently small, about 8 microns orless, to adequately prevent the passage of liquid through the menbrane.In addition, the pore size of this filter membrane should also besufficiently small to filter out most bacteria, and a desirable meanaverage pore size range is from about 0.1 to about 3.0 microns,preferably about 0.22 microns.

Because polytetrafluoroethylene melts at about 400° F., which issignificantly higher than the approximate 350° F. at which the preferredhousing material melts, which makes simple heat sealing difficult, thepresent invention includes two new and unique ways for attaching thefilter membrane to the cap. The preferred method of attachment is bestshown in FIG. 5. In this construction, the hydrophobic membrane isprovided with a thin laminated layer of backing material 42 of fibrousconstruction. The membrane and backing layer must be sufficiently largeto extend fully across the cover the vent ports 38 and overlap the areaof the cap around the vent ports, but because of the position-sensitivelocation of the vent ports and the material used, the membrane 40 may bemuch smaller than the liquid-wetting membrane 24 which extends acrossthe entire filter. In fact, the liquid-repellent membrane may be lessthan one-half of the area of the liquid-wetting membrane andsatisfactory venting is obtained with a liquid-repellent membrane lessthan one-fifth the size of the liquid-wetting membrane.

The liquid-wetting membrane is positioned with the backing layer betweenthe membrane and the cap. A heat sealing head, not shown, is thenpressed against the membrane. The sealing head has a annular heatedcontact surface of appropriate size to engage at least a continuousportion of the peripheral membrane area overlapping the cap. The head isheld in contact with the membrane until the portion of the captherebeneath is heated enough to melt and become sufficiently liquid toflow or migrate into interstices between the fibers in the fibrousbacking. The heat sealing head is then removed and the plastic cooled,the portions of plastic in the interstices in the fibrous backingforming a secure, watertight seal with the membrane and backing.

In the preferred embodiment, the backing material is made of non-wovenpolyester fiber, which is available already laminated to thepolytetrafluoroethylene filter membrane, and is referred to as a Reemaybacking, type LI0931, from the Gore-Tex Company of Elkton, Md. U.S.A. Ithas been found with these preferred materials that a satisfactory sealis created between the filter and a methylacrylic cap when the heatsealing head is heated to about 350° F. and pressed against the filtermembrane with a pressure of about 80 psi for approximately 3 seconds.Other combinations of temperatures, times, and pressures may also beused, but the example above is the preferred. Alternatively, ultrasonicwelding techniques may be used to melt the plastic cap until plasticfills a continuous portion of the interstices in the fibrous backing.

Another alternative means for attaching the liquid-repellent filtermembrane 40 over the vent ports is shown in FIG. 9. In FIG. 9, as inFIG. 5, the filter membrane is of sufficient size to cover the ventports 38. A continuous adhesive tape 44 overlaps the peripheral edge ofthe membrane and the inside surface of cap adjacent to the membrane. Foruse in intravenous solutions, the tape is preferably of medical grade,non-toxic and compatible with the fluid being filtered. One such tapewould probably comprise an acrylic carrier with acrylic adhesive forcompatibility with the solution being filtered. Medical tapes which havebeen considered for attaching the filter are available from 3M Companyof St. Paul. Minnesota, U.S.A. under the trademarks BLENDREM andTRANSPORE.

In actual construction, the membrane 40 is disc-shaped and the tape hasa ring or donut configuration. The membrane is first mounted against theadhesive side of the tape and then the tape and membrane are mountedagainst the inside surface of the cap, covering the vent ports. A heatsealing head like that described above, with an annular heating surfaceengageable against the tape, is then pressed against the tape and heldin place with sufficient pressure and for sufficient time for theadhesive to become better activated and thereby provide better adhesionwith the membrane and housing. It is also believed that heating relievesstresses in the acrylic tape which may occur during fabrication. It ispresently understood that a better tape seal is obtained and stressesare relieved using a sealing head heated from about 150°-200° F. andpressing it against the tape at a pressure of about 50-70 psi forapproximately 2-3 seconds. Whether the liquid-repellent membrane isattached to the cap by tape or mechanical bond, the cap and base portionare preferably sized so that the liquid-repellent and liquid-wettingmembranes are at least five millimeters apart when the housing isassembled.

Although the liquid-repellent membrane 40 prevents liquid from escapingthrough the vent ports, if the filter is employed in a situation whichpermits less than atmospheric pressure to exist in the filter housing,i.e., a negative pressure differential, outside air will tend to enterthrough the liquid-repellent membrane and enter the solution beingfiltered. In such a situation, if the hydrophilic filter membrane 24 isnot intact, for whatever reasons, outside air entering the solutionwould be conveyed directly into the patient, possibly resulting inembolism. To permit trapped gas to vent from the housing, and yetprevent outside air from entering during a less than atmosphericpressure situation in the housing, an automatic vent control means isprovided which is sensitive to the differential pressure between theinside of the filter housing and the ambient atmosphere. In theillustrated embodiment of this invention, the automatic, pressuresensitive vent control means is provided in the form of an umbrellavalve, generally at 46, which may be secured over the vent ports on theoutside of cap 14. Referring to FIG. 4, the umbrella valve has agenerally circular canopy or dome 48 which, in an unstressed position,extends at an acute angle to a centrally located stem 50. The stem has amiddle bulbous portion 52 beyond which is a tapered shaft portion 54.The entire umbrella valve is one piece, integral construction and may bemolded from rubber, preferably natural rubber, although any sufficientlyflexible and resilient material that is compatible with the filter fluidmay also be used.

The vent ports 38 are located in a raised cylindrical portion 56 of thecap 14. The cylindrical portion has a flat internal shoulder 58surrounding the outer edge of a central cylindrical recess or depressiondefined by the wall surface 60. Referring briefly to FIG. 3 for clarity,the four vent ports are equally spaced, at 90° intervals around theperipheral wall 60 of the central recess.

The canopy 48 of the umbrella valve is of sufficient size that itsperiphery overlaps the inside shoulder 58, surrounding the vent ports 38and completely covering them.

The umbrella valve 46 is mounted over the vent ports 28 by securing thestem to attachment means such as a central bore 62 in the bottom of thecylindrical recess. The bore is of substantially smaller diameter thanthe bulbous portion of the stem, which acts as an anchor to hold theumbrella valve in place. The tapered shaft portion 54 of the stem isinserted into the bore and pulled from the inside of the cap until theelastic bulbous portion passes through the bore and resumes its normalshape on the inside of the cap. The distance between the inside edge ofthe central bore 62 and the shoulder 58 is less than the length of stembetween the canopy 48 and the bulbous portion 52. Thus, when the bulbousportion is drawn through the bore. the elastic stem is stretched,exerting an axial force on the canopy and drawing it into a normaltight, sealing contact against the shoulder. The tapered shaft portionis then snipped or cut from the bottom of the bulbous portion so as notto interfere with the liquid-repellent filter membrane 40. A flatcontact surface 64 is provided along the underside of the peripheralportion of the canopy to aid in preventing sealing contact with theshoulder.

With this construction, should the liquid-wetting filter fail, theelastic umbrella valve seals the vent ports against the entry of outsideair into the filter housing, and a vacuum inside the housing, ornegative pressure differential between the housing and the ambientatmosphere only serves to draw the canopy into a tighter seal over thevent ports. While preventing outside air from entering, the flexiblecanopy nevertheless is sensitive to the pressure of gas accumulatedinside the housing and permits such gas inside the housing to ventoutwardly when its pressure is sufficiently greater than the atmosphericpressure i.e., a positive pressure differential. When entrained gas isseparated from liquid by the liquid-wetting filter membrane, it risestoward the vent ports, which are located in the upper part of thehousing. As particularly illustrated in FIG. 6, when the gas in the ventports reaches sufficient pressure, it causes the flexible canopy toinvert or flex upwardly, away from the shoulder 58 and vent ports,thereby permitting the separated gas to vent to the ambient atmosphere.After the gas has vented, the resilient canopy resumes its sealingposition over the vent ports, preventing any outside air from enteringthe housing.

As noted earlier, one use for the filter is in hanging position in aparenteral solution administration set. The filter would be positionedbelow a parenteral solution container for example, about 15-18 inchesbelow the container. What a 15-18 inch head of liquid, the pressure ofentrained gas separated from the liquid will probably be somewhat morethan 1/2 psig, which is a positive differential pressure between theentrained gas and the ambient atmosphere. In such an application, theumbrella valve is preferably constructed to selectively releaseseparated gas having a pressure of about 1/2 psig. The release pressureof the umbrella valve is a function of many variables, including thevalve material, the thickness of the canopy, the tension in the stem andthe degree of angle between the canopy and the stem, all of which may bereadily determined by one skilled in the art after reading thedescription of this invention. However, as an example, one filter whichreleased trapped gas at about 1/2 psig had the following features: inthe cap, the distance between the inside edge of the bore 62 and theshoulder 58 was about 0.13 inches, the umbrella valve was made ofnatural rubber and had a canopy about 0.30 inches in diameter and about0.03 inches thick, at an approximate acute angle of 61° with the stem;the distance between the top of the canopy and the center of the bulbousportion of the stem was about 0.192 inches; and the stem was uniformly0.07 inches thick between the canopy and the bulbous portion.

On the other hand, the present filter is not limited to the particularapplication described above but may also be employed at a lower positionin the administration set or in an "extension set" which is at about, oronly slightly higher than, the level of venous entry. Under suchconditions, a substantially greater head of liquid is created above thefilter providing positive differential pressures of up to about 3 psig.So, depending on the particular application, the umbrella valve may beproportioned and assembled to vent at any selected pressure betweenabout 1/2 and 3 psig. An umbrella valve with a higher release pressureusually also has a better sealing engagement between the umbrella canopyand the shoulder 58.

When the filter is used in systems which may not create sufficient gaspressure for automatic venting, the cylindrical recess defined by thewall surface 60 beneath the canopy provides a means for manually ventingthe filter housing. As described earlier, the vent ports are spacedaround the recess. More particularly, and referring briefly to FIG. 3,the wall of the recess generally bisects each vent port. Now looking toFIG. 7, by manually depressing the center of the canopy 48 into therecess, a fulcrum-type engagement between the canopy and the edge of therecess causes the canopy to flex upwardly, thereby exposing at least aportion of the vent ports to the ambient atmosphere and permitting theseparated gas to vent. Upon release of the umbrella it resumes thenormal, sealing position. Because the umbrella valve is firmly anchoredto the cap 14 by the bulbous portion and the canopy is surrounded by thewalls of the raised portion of the cap, the umbrella valve is not easilyremoved or disabled so that the liquid-repellent filter would always beexposed to the ambient atmosphere.

This manual venting feature may be useful even in a filter in which itis desired, for whatever reason, that the umbrella valve notautomatically vent. For example, an umbrella valve may be constructed ofsuch proportions to provide an expecially tight seal between the canopyand the shoulder. Such a valve may not automatically vent under therelatively low head pressures in an administration set. However, thepresent invention permits simple and effective manual venting of such afilter, for example, for priming the administration set.

It can therefore be seen from the description above that in accordancewith the present invention a new and improved filter is provided forseparating gas from liquid and venting the gas to the atmosphere. Thefilter is relatively simple and inexpensive to assemble. Theliquid-repellent filter membrane with a fibrous backing for forming amechanical bond with the plastic cap provides a much simpler andefficient method for attaching the filter, as does also the tape ringconstruction. A pressure sensitive vent control, such as the flexibleumbrella valve, may also be employed to permit use of the filter in avariety of systems by providing an effective means for permitting gas toautomatically vent while blocking the entry of outside air into thehousing. And for gas of insufficient pressure to automatically ventthrough the umbrella type valve, the recess below the canopy permitsmanual flexing of the canopy to vent separated gas from the filter.

Although described for purposes of illustration in terms of thepreferred embodiment, which is particularly useful for in-linefiltration of parenteral solutions, the present invention is alsointended to include such changes or modifications as come within thefollowing claims, some of which changes or modifications may beimmediately apparent and others of which may occur only after somestudy. For example, an umbrella valve configuration is not the only typevent control means which may be used, but other type systems, e.g.,baffles or other types of controls which are sensitive to thedifferential pressure between the inside of the filter housing and theambient atmosphere may also be used.

That which is claimed is:
 1. In a gas separating and venting fluidfilter comprising: a housing including inlet opening means and outletopening means, liquid-wetting filter means disposed in a flow pathbetween said inlet and outlet opening means to permit the passage ofliquid only, vent opening means in said housing on the upstream side ofsaid liquid-wetting filter means to vent gas from the housing, andliquid-repellent filter means in the path of venting gas to permit thepassage of gas only and to prevent the escape of liquid from saidhousing, the improvement comprising: automatic vent control meanscarried by said housing, said vent control means being sensitive to thedifferential pressure between the interior of said housing and theambient atmosphere and operable under a positive differential to permitventing of separated gas from the vent opening means to the ambientatmosphere, and being further operable to prevent the admission of gasfrom the ambient atmosphere into the vent opening means, and saidliquid-repellent filter means being generally planar and being mountedon said housing by a continuous medical grade adhesive tape whichoverlaps the peripheral edge of said filter means and the portion ofsaid housing adjacent said peripheral edge.
 2. A gas separating andventing filter comprising: housing means having walls defining aninterior chamber, liquid-wetting filter means carried in said housingand dividing said chamber into first and second subchambers, inletopening means in said first subchamber for introducing fluid thereinto,said liquid-wetting filter permitting passage of liquid only into saidsecond subchamber, outlet opening means in said second subchamber forpermitting the passage of filtered liquid therefrom, vent opening meansin said first subchamber communicating with the ambient atmosphere forventing gas collected in said first subchamber, liquid-repellent filtermeans carried by said housing means in said first subchamber anddisposed in the path of said venting gas to permit the passage of gasbut to prevent the escape of liquid through said vent opening means,said liquid-repellent filter means being generally planar and beingmounted on said housing by a continuous medical grade adhesive tapewhich overlaps the peripheral edge of said filter means and the portionof said housing adjacent said peripheral edge, and automatic ventcontrol means carried by said housing, said vent control means beingsensitive to the differential pressure between the interior of saidhousing and the ambient atmosphere and operable under a positivepressure differential to permit venting of separated gas from said firstsubchamber to the ambient atmosphere and being further operable toprevent the admission of gas from the ambient atmosphere into the ventopening means.